I am studying thrombosis and morphology of mechanical heart valve prosthetics. Mechanical heart valve prosthetics made with pyrolytic carbon are widely used (millions of patient-years experience) although thrombotic and embolic complications remain significant (1-5% of patients per year). The mechanisms of the biocompatibility of these devices is not fully understood. Recently, we have demonstrated that the matenal used in these devices is not as passive to blood platelets as has been thought since the 1970s. In these studies we also have revealed that the 3-D structure of the pyrolytic carbon is considerably more complex that previously described. Low voltage SEM imaging enables the examination of specimens with better signal to noise ratio than obtained with conventional SEM and with less beam damage due to a restricted penetration volume of electron beam. Using the Hitachi S-900 at the MIR we are able to examine this low density, very rough material and obtain information never before documented. In the current project we will use LV-SEM to examine the mechanisms of platelet adhesion, activation, and thrombus growth on heart valve components.